Hi Ultima,
Really appreciate all the advice and help you've rendered to us
on this forum!
I have some questions on today's paper:
1. For the question on carbon monoxide and how it inhibits the
transport of blood, will it suffice to just elaborate on the
strength of the ligand, formation of dative bonds and ligand
exchange? Or is further elaboration on "binding irreversibly"
required?
2. For the question on the reagents and conditions (for
nucleophilic addition), Step 1, is it required to indicate 10-20
deg C for HCN and NaCN/KCN trace catalyst, or is room temperature
sufficient? This is because I recalled some reference literature
not specifying temperature conditions.
3. For the question on why a brief exposure to light is
sufficient, will it suffice to elaborate on how the presence of
reactive free radicals due to initial homolytic fission of Cl-Cl
propagates a chain reaction or must we elaborate on the
regeneration of free radicals?
4. For the question pertaining to the reactivity of Br2 with H2
compared to Cl2 and H2 (with reference to Data Booklet bond
energies), will it suffice to discuss about the exothermicity of
the formation of H-Br and H-Cl (based on Bond Energies) only or are
candidates expected to discuss the overall enthalpy changes of the
reactions? I felt that since the latter requires a consideration of
the Bond Energies of Br-Br and Cl-Cl, and since the BE of Br-Br is
less endothermic than that of Cl-Cl, does it really explain why Br2
is less reactive than Cl2 if its bonds are easily broken?
Thank you for your time!
U're welcome ;)
Q1. For such questions, Cambridge provides a bit of leeway, in that
out of say, 4 acceptable key marking points for a 3 mark question,
you can score the full 3 marks if you wrote any 3 of the 4 marking
points correctly. So it's still possible to score full marks
without mention of "binds permanently / irreversibly". No
worries.
Q2. When in doubt, it's always safer to write it in. But in this
particular case, unless Cambridge specifically asked for
"conditions" or "temperature", otherwise just for "reagents",
Cambridge will accept either "HCN" or "HCN and CN-". So no worries,
but for P3, do yourself a favor and just play it safe when in doubt
:)
Q3. Because the question specifically mentioned, "The rate of
reaction increases, explain why", hence to get full marks for this
question, you need to specify "Due to the large no. of different
possible propagation reactions, free-radical concentrations will
continue to increase, hence increasing rate of reaction, until the
rate of termination reactions catch up" OWTTE (Or Words To That
Effect). But you should still get the marks for the rest of the
question.
Q4. To obtain full marks, other than quoting Data Booklet bond
enthalpies (or energies, for A level Chem purposes they're
interchangeable), you need to mention that the decrease in
magnitude of exothermic H-X bond formation enthalpy outweighs the
decrease in magnitude of endothermic X-X bond dissociation
enthalpy, going from Cl to Br. So yes, best answer is, as you say,
take into consideration both the bond dissociation and bond
formation enthalpies, leading to the conclusion that the overall
enthalpy change of reaction to generate H-Cl is more exothermic
(hence more thermodynamically favorable) compared to H-Br.